I've only connected once through the east coast for SFO/LHR in BOS...never again. It's the worst of all worlds, slicing the trip in roughly 1/2 for each leg with a layover in the middle.

Although I agree about the WX in IAH, I've only read the horror stories about IAH immigration on FT, and it is quite possible in the winter, to have a multiple-hour ground delay IAH/SFO.

So, if it were my money, I'd go through YVR. I had to go the other way once because of UA's systems melting down, and the transit in YVR outbound was trivial. I'd expect that you'd clear US border formalities in YVR after CA immigration for the transit, but honestly don't know what that would entail.

My bottom line is that I'd rather have one very long flight, and one as short as possible. The suggestion about re: FRA also fits the bill, especially because long-haul intl flights won't generally be affected by the ATC delays. SFO domestically in the winter months is a complete crap-shoot, even day-to-day, much less 5 months out.

One additional thought -- if the YVR/SFO connection is on SkyWest, I'd pass on the routing. If on AC/UA, you'd have a fighting chance.